1. Field of the Invention
This invention relates to an optical nondestructive testing method of composite materials, and in particular to an optical nondestructive testing method of laminate-structure composite materials.
2. Prior Art Statement
A composite material is a material that is obtained by uniting two or more materials or two or more layers by some physical or chemical method. Compared to simple materials, composite materials have excellent lightness and strength, and because of this composite materials have recently come into use as construction materials for aircraft and the like.
However, any internal defect, such as a disbond, for example, causes a loss of the properties of the composite material, which in the case of an aircraft application, for example, could result in a major accident. This being the case, nondestructive testing of composite materials is of major importance.
Optical interferometry is one of the known optical nondestructive testing methods of composite materials. In optical interferometry, the composite material is vibration stressed and holographic or speckle pattern interferometry is used to find the shape of a disbond from the resonance mode of the disbond site.
L. A. Kevsch, for example, has pointed out that when the shape of a disbond is a circle (of radius r), the depth d of the disbond with respect to resonant frequency f is f=kd/r.sup.2 (p. 306 Holographic Nondestructive Testing edited by R. K. Erf, Academic Press (1974)). Here, constant k is k=.beta.,.sqroot.[E/3.rho.(1-V.sup.2)],.beta. is a constant decided according to the resonance mode, E is the Young's modulus, .rho. is density and V is Poisson's ratio.
Conventional optical testing methods enable the shape of disbonds and other such defects in composite materials to be found, but because the usual objective was the shape of a defect, it was not possible to ascertain the depthwise location of the defect in the material. In order to repair defects in composite materials it is necessary to know the depth at which the defect is located as well as its shape. As such, there has been a demand for the development of a testing technique that would enable both the shape and the depth of a disbond or other such defect in a composite material to be found.